Posted
by
kdawson
on Friday September 04, 2009 @09:55AM
from the go-north-young-man-and-keep-on-going dept.

Thorfinn.au sends along big physics news: magnetic monopoles have been detected at low temperatures in "Dirac strings" within a single crystal of Dysprosium Titanate. Two papers are being published today in the journal Science and two more on arXiv.org, as yet unpublished, provide furtherevidence. "Theoretical work had shown that monopoles probably exist, and they have been measured indirectly. But the Science papers are the first direct experiments to record the monopole's effects on the spin-ice material. The papers use neutrons to detect atoms in the crystal aligned into long daisy chains. These daisy chains tie each north and south monopole together. Known as 'Dirac strings,' the chains, as well as the existence of monopoles, were predicted in the 1930s by the British theoretical physicist Paul Dirac. Heat measurements in one paper also support the monopole argument. The two, as yet unpublished, papers on arXiv add to the evidence. The first provides additional observations, and the second uses a new technique to determine the magnetic charge of each monopole to be 4.6x10-13 joules per tesla metre. All together, the evidence for magnetic monopoles 'is now overwhelming,' says Steve Bramwell, a materials scientist at University College London and author on one of the Science papers and one of the arXiv papers."

The kind of monopoles they talk about in relation to the LHC would be a new particle of somesort which has a single "magnetic charge" like the electron has an electric charge, for some complicated reasons some people believe that the existance of even one such particle in the universe would be a pretty big deal and possibly a bad thing if we created one.

The kind of monopoles created here are configurations of molecules(?) in a lattice that forms something called a spin glass, essentially it allready has lots of little bar magnets in it allready. What's interesting is they can apparently create monopole pairs, like the electron-hole pairs in a semiconductor. The behaviour of these should still match those of a pair of magnetic monopoles which is where all the dirac string buisness is from.

A magnetic monopole is to a magnetic field what an electron is to an electric field.

This will, amongst other things, mean that Maxwell's equations become more symmetrical.

div D = rho; div B = 0

Will become

div D = rho_e; div B = rho_m

And there will be a magnetic current term for curl H.

It's long been known that if a magnetic monopole exists then charge must be quantized.

I've not looked at any of the papers but I'm interested to find out if they've got a mass estimate for them. Last I remember reading about this they were expected to be heavy (uranium nucleus sort of heavy) but I don't recall if that was an extrapolation from their non-detection or whether there was a more fundamental reason for them needing to be so massive.

Now I've scanned one of the papers I see that they're not detecting the sort of magnetic monopole I was thinking of (i.e. a new sub-atomic particle)

Instead they've detected the equivalent of a charged molecule.

They give an analogy of the disassociation of water into H3O+ and OH-. They claim to have done the same thing with magnets - ending up with a disassociated north and south pole.

So their work doesn't appear to give any clue to the mass of a magnetic monopole particle. But AFAICT they have still created a type of magnetic monopole, exactly the same way as a proton is an electric monopole even though it has an internal structure.

You could just as well ask: "how can an electric field line just stop somewhere?", and thereby conclude that there can be no such thing as an "electric monopole" (a positively- or negatively-charged particle). As long as the universe has no net electric or magnetic charge, all lines will terminate somewhere. If the universe did have a net charge the point is subtle, but that's irrelevant: the paper talks above pairs of opposite-pole monopoles created together, like a particle and its antiparticle. So this argument doesn't hold water.

Monopoles aren't impossible in principle (it would just be an extra term in Maxwell's equations) and are predicted in some theories, but fundamental-particle monopoles have never been observed. The summaries of this paper are confusing a lot of people: the authors are describing a crystal system with excitations that look like monopoles. They are NOT describing discovery of a new fundamental particle, but rather a new kind of solid-state phenomenon.

Having read at least one of the arxiv articles, it is clear to me that the authors have NOT detected magnetic monopoles, and don't actually claim that they have.
They claim that a certain type of ordering in a very specific crystal at very low temperatures BEHAVES AS IF it was a magnetic monopole - it's an analogy at best. The energy required to trigger the effect is minute, so they can "see" lots of MMAs (magnetic monopole analogs [my terminology]), and hence study what would happen if lots of REAL MMs existed in some other situation. They confirm that setting up Maxwell's equations to include a monopole shows the same sorts of behavior as what they see.
But a real, isolated magnetic monopole? Not this time......

The spin ice state is argued to be well-described by networks of aligned dipoles resembling solenoidal tubesâ"classical, and observable, versions of a Dirac string. Where these tubes end, the resulting defect looks like a magnetic monopole.

They've managed to create the microscopic equivalent of a long skinny magnet or a long bendy solenoid: a set of dipoles aligned end-to-end, which acts just like a string with two "monopoles" at the ends.

While this is an interesting microscopic state of matter, from the "discovering monopoles" point of view it doesn't seem fundamentally different than the macroscopic description of magnet "poles" that has been well understood for over a century (and observed for a lot longer than that). I call hype.

The Dirac string is not real, but is really just a failure of the coordinate system. Coordinate systems are always failing: just stand at the North Pole and ask which direction is South. All directions are, and the coordinate system is broken at that point. That's fine, it works everywhere else and we jsut remember to be careful in the rare cases where we are asking for directions at the North Pole. There is no fundamental breakdown of space and/or time going on, it's just because we chose to impose a silly coordinate system onto the physical world. The fact the it breaks down at the North and South poles is also a red herring - we just chose to make the polar axis the same as the axis of rotation of the earth.

The coordinate system used to simultaneously describe electric and magnetic charges is also broken, and the Dirac string is really just a way of fixing up this breakage. We imagine that one unit of magnetic flux arrived through a very narrow tube at the monopole and then spewed out in all directions (think toddlers or teenagers at this point). The tube is not real, it is just a way of patching up the failure of the coordinate system. In the same way that the Poles as points of failure is our choice, the direction that the tube arrives at the monopole is also an artifact of how we set up the coordinate system.We can change the direction that the tube arrives at the monopole from transparently using what is technically known as a gauge transformation, but let's not worry about that here.

The tube is not real, so we must not be able to detect it. This leads to the concept of quantisation of electric charge. Normally, if you take a tube carrying g units of magnetic flux and then take an electrically charged particle round it in a circle, the wavefunction of the charged particle will change by a complex phase exp(i.theta) where theta is proportional to the product of q (its electric charge) and g. You can detect this phase using a quantum-mechanical interference experiment, if you feel the urge. If the Dirac string is to remain physically unobservable, no interference effects must be seen so the phase rotation must be a multiple of 2.pi, because exp(i.2.pi)=1.

So, we know if there is one magnetic monopole anywhere in the universe then q.g = 2.pi.n (where n is some integer) so that the Dirac string (a mathematical fix for a choice of broken coordinate systems) remains just a theoretical trick and not observable physics. We must then have that the electric charge of every particle in the universe is some integer multiple of e = 2.pi/g, where g is the magnetic charge of that monopole.

Whether you consider the smallest unit of electric charge to be the charge on the electron or the charge on a free quark (one third of this) doesn't matter. We do observe that electric charge is quantised (i.e. integer multiples of some base amount) and magnetic monopoles as fundamental particles provide a relatively elegant solution as to why this is true.

As I have stated in response to circletimessquare elsewhere in this discussion, there are a few good reasons to believe magnetic monopoles might exist. I remember circletimessquare's sig, and remember him or her making good posts in the past, but it's clear that he or she does not understand electricity and magnetism very well.

Classical electric and magnetic fields vary in a coordinated way between different Lorentzian reference frames. So where one observer might only observe an electric field with no magnetic field, an observer in a different reference frame moving at a constant velocity with respect to the first observer's frame might see a combination of electric and magnetic fields. Electricity and magnetism are different aspects of a single force, believed to be one of the four "fundamental" forces. It is called, shockingly enough, the electromagnetic force. That's one reason to believe that since electric "monopoles" (charges) exist, magnetic monopoles might too.

There are electric dipoles, which are made of opposing electric "monopoles" (charges). Why couldn't magnetic dipoles also be made of opposing magnetic monopoles? That's another reason to believe magnetic monopoles might exist.

Dirac didn't just think magnetic monopoles might exist for no reason. He discovered in his calculations that the existence of magnetic monopoles would automatically lead to the quantization of electric charge. Since all electric charge observed in nature is quantized (in integer multiples of the electron charge for free particles and, we believe, in integer multiples of 1/3 of the electron charge if we include particles that are not observed "free"), we have yet another reason to believe there might be magnetic monopoles.

Very smart folks with Ph.D.s in physics have been looking for magnetic monopoles in creative ways for a very long time. In another post in this discussion, I mentioned Professor Henry Frisch of the University of Chicago. These people aren't just looking for magnetic monopoles to do something crazy. They're doing it because their deep understanding of the theory and the experimental data leads them to believe magnetic monopoles might exist.

Classic case of science journalists overblowing a mundane result. Yes, connected quasi-monopoles are interesting. they are visible in any conducting medium. But there's a HUGE difference between a quasi-monopole that is at the end of a finite-length, shielded dipole and a true monopole that actually violates the magnetic divergence-free condition.

In solar physics we call such things "unipoles" to distinguish them from the infinitely harder-to-find "monopoles". Unipoles are all over the surface of the Sun, because the conductive interior hides the field lines that connect opposing unipoles.

It is disingenuous at best and downright deceptive at worst to call the HZB result "evidence for magnetic monopoles", because it ain't.

The only plausible true magnetic monopole detection ever was still in Blas Cabrera's instrument at Stanford in the 1980s. It was never replicated, so it is unknown whether they exist but are extremely rare (and Cabrera was just lucky) or whether his detector glitched.

For 30 years, physicists have believed that the universe is made up of tiny vibrating dimensional strings which only they are clever enough to understand. A fine idea, except it turns out not even they are clever enough after all. Nevertheless, they persist in this belief because the mathematics is beautiful.

It is incorrect to say "physicists have believed." It would be correct if you were to say "some physicists," or even better, "a small minority of physicists." String theorists certainly put a lot of stock in string theory; but even among that group of physicists, not all of them believe it's right so much as they think it's an idea worth working on. And at any rate, string theorists make up a tiny fraction of the community of physicists. Outside of that community, there's a lot of physicists who think it's hogwash, a lot of physicists who think it's uninteresting as long as it's so far divorced from the experimental realm (including myself), and a lot of physicists who simply don't care one way or the other because their work is in so separate a domain that they don't have a dog in that hunt.

I mention this because the overall premise of your post -- that physics (or, more accurately, physics research) is becoming more and more divorced from experiment -- is not borne out by my experience as a professional working physicist. Even among string theorists, of which I've known a fair number, I've never met any physicist who thinks there's virtue in untestable conjecture. They simply believe that if they work hard enough and are clever enough, they'll come up with effective ways to test string theories that are reachable by experiment or observation. They may be wrong about that (and whether they are or they aren't wrong, until they do come up with some way to test it, I'm not interested); but all the string theorists I've known understand quite well the importance of experiment and observation. They aren't simply believers. And at any rate, string theorists make up a small fraction of the community of research physicists.

There's not as much difference as you think. True, magnetic fields are not caused by "charges" as far as we know, unlike electric fields. But there's no theoretical reason why that's not possible. Electric fields can be caused by things other than static electric charges - for example, changing magnetic fields or (hypothetically) a magnetic current - a flow of magnetic monopoles.

The electron does indeed have an electric monopole and magnetic dipole, but there could conceptually be a magnetic monopole - we just haven't seen any. The monopole charge density would be a scalar, just like the electric charge, so I don't know what you're getting at with your "unit vector without a direction" remark.

Thanks for posting this, though I realize a lot of people don't have access.

The abstract of the article in Science actually makes matters quite clear:

While sources of magnetic fields--magnetic monopoles--have so far proven elusive as elementary particles, several scenarios have been proposed recently in condensed matter physics of emergent quasiparticles resembling monopoles. A particularly simple proposition pertains to spin ice...well-described by networks of aligned dipoles resembling solenoidal tubes--classical, and observable, versions of a Dirac string. Where these tubes end, the resulting defect looks like a magnetic monopole.
[emphasis mine]

This makes it clear that they have not discovered a fundamental particle that is a monopole, which people have been searching for for a while. What they've discovered is a material where under certain conditions you can model the behavior as though there were monopoles present, but it's an imaginary construction, not an actual particle; that's what they mean by quasi-particle. As someone else mentioned, this is similar to how you can describe as hole, where an electron is missing in a semiconductor, as though it were a positive charged particle moving around in the material. In this case, they have a long series of aligned dipoles that they're saying is similar to a very long solenoid. If you're outside the solenoid near one of the ends it just looks like a monopole (because all the magnetic flux going the other day is confined to the narrow region inside the solenoid).

These are simply sets of atoms that, together, act like monopoles. What has been discovered is not a single particle with one pole. It is a place inside a material that acts like a monopole. Real 'Dirac strings' connecting real monopoles are not long chains of molecules, these long chains of molecules simply act like Dirac strings. Please. This is the most misleading title and summary I've ever read here, and that is saying A LOT.

It is not possible to create a true monopole from dipoles, because any "g'zinta" field lines to your favorite point in space have to matched by an equal number of "g'zouta" field lines from the same place.

These spin-glass phenomena are only quasi-monopoles: all the "g'zinta" field lines are squished into a small tube, leaving the "g'zoutas" free to splay out almost like a true monopole. But the divergence is still zero (there are no field line endpoints).

Compare to a spray nozzle that sprays water in all directions from a garden hose. If the spray is broad and strong enough, it might sort of hide the hose itself, so that you could convince your kid brother that the nozzle is a magical water-creator (i.e. that the flow through the nozzle has positive divergence). But in fact, there's a garden hose feeding the nozzle, and every bit of water that comes out through the nozzle is balanced by an equal bit coming into the nozzle from the hose.

In that analogy, your nozzle is interesting because the spray pattern is similar to the pattern from a mythical water-creator, but it still won't solve the problem of drought in California, which a true water-creator would.

In a box approximately 1nm on a side there is a north pole with no matching south pole. So there are magnetic field lines flowing out of the box with no matching field lines flowing in. Of course "over there" there is a south pole which has field lines flowing in without field lines flowing out.

Except, from my reading above, it seems that the matching "monopoles" are connected by a long series of aligned dipoles resembling solenoidal tubes, so there is mag field in the dipoles flowing into the box. Sure the "in" field occupies a very small area, but in/out still balances. Is this incorrect? If not, I don't see how this is evidence of the existence of monopoles.

And, as an aside, given the parallel you draw with conservation of charge, is there any corresponding persistent and unique connection between (e.g.) an electron and positron pair after they are created?